1. Field of the Invention
The invention relates to the leak testing of automotive vehicle fuel tanks and associated evaporative emissions control systems. More particularly it relates to a method for testing such systems which corrects for a fundamental problem with previous methods: the lack of compensation for variations in the test conditions of liquid fuel temperature and volatility.
2. Background Information
Several previous inventions have dealt with the problem of leak detection from automotive fuel tanks and associated evaporative emissions control systems. Rogers, in U.S. Pat. No. 5,369,984, described a method for “testing of tank integrity of vehicle fuel systems.” Fournier, in U.S. Pat. No. 5,425,266, detailed an apparatus and method for the “non-intrusive testing” of such systems, employing an annular device to produce a seal between a pressure control unit and the fuel tank filler neck into which the pressure control unit was inserted. Kammeraad et al., in U.S. Pat. No. 5,507,176, described a tester utilizing an adapter fitting onto the access port for filling of the fuel tank, which allows the fuel cap to be tested in tandem with the rest of the evaporative system. Kolb, in U.S. Pat. No. 5,509,296, described a system for applying pressure to a fuel tank by placing a sleeve over the fuel tank filler neck. Chirco et al., in U.S. Pat. No. 5,644,072, described a system of pressure regulators and pressure sensors for pressurizing and measuring the pressure decay of a fuel tank. Lycan et al., in U.S. Pat. No. 6,289,722, described a helium leak tester for vehicle fuel tanks, intended for use in finding small leaks in fuel tanks prior to their installation in motor vehicles. Harris, in U.S. Pat. No. 6,327,898, described a system employing a device measuring differences in pressure from the evaporative system and the outside environment, in order to generate a pass-fail decision.
All of the above inventions involve some variant of the basic procedure of pressurizing a fuel tank and observing the resulting pressure decay to make a pass-fail determination—i.e., does the system have a leak sufficiently large that repair is necessary. A basic problem with all of the above inventions is that these tests do not consider all of the considerable variation in fuel temperature and volatility under which the vehicle is leak-tested, which can strongly influence the test results. High-temperature or high-volatility fuel is prone to sudden changes in fuel vapor pressure due to the increased vaporization rate of the fuel; these changes can impair or mask entirely the ordinary release of pressure through a leak. Thus, if the pass-fail decision cutpoint is not compensated for these variables, a problem arises: a vehicle with a given size leak may pass one test and fail another, due to differences in test conditions (irrespective of any repair). Such an outcome is most unfavorable in a governmental regulatory environment, where such tests need to produce reliable, consistent outcomes.